Almost all known mineralogical forms of MgSO 4 are hydrates. Epsomite is the natural analogue of "Epsom salt". Another heptahydrate, the copper -containing mineral alpersite (Mg,Cu)SO 4 ·7H 2 O,  was recently recognized. Both are, however, not the highest known hydrates of MgSO 4 , due to the recent terrestrial find of meridianiite , MgSO 4 ·11H 2 O, which is thought to also occur on Mars. Hexahydrite is the next lower (6) hydrate. Three next lower hydrates—pentahydrite, starkeyite, and especially sanderite are rare. Kieserite is a monohydrate and is common among evaporitic deposits. Anhydrous magnesium sulfate was reported from some burning coal dumps.